Non-violence is a supreme virtue. The Bhagavad Gita describes it as a divine quality (16.2) and as a form of physical austerity (17.14). Non-violence consists in not harming others by thought, word, or deed. A spiritual seeker takes care to be free from the tendency to harm others in any way, whether for selfish interest or even for his self-preservation. Says a Sanskrit proverb, "Helping others is meritorious; harming others is sinful."
Non-violence is based on love: Non-violence can be practiced only when we have love for others. We cultivate this love by remembering that we are all children of God. Says Patanjali in his Yoga Sutras, "When we are established in non-violence, all enmity ceases in our presence." (2.35) True non-violence includes love for animals. Swami Vivekananda explains: "As with some, [non-injury] does not simply mean the non-injuring of human beings and mercilessness towards the lower animals; nor, as with some others, does it mean the protecting of cats and dogs and feeding of ants with sugar-with liberty to injure brother-man in every horrible way! It is remarkable that almost every good idea in this world can be carried to a disgusting extreme. A good practice carried to an extreme and worked in accordance with the letter of the law becomes a positive evil."
Non-violence is not a result of weakness: Non-violence is a quality born of sattva, or calmness. It has nothing to do with weakness, a trait of tamas, or inertia. Non-violence can be a virtue only when we have the capacity to strike back but don't. In the words of Swami Vivekananda, "The karma-yogi is the man who understands that the highest ideal is non-resistance, and who also knows that this non-resistance is the highest manifestation of power in actual possession, and also what is called the resisting of evil is but a step on the way towards the manifestation of this highest power, namely, non-resistance. Before reaching this highest ideal, man's duty is to resist evil; let him work, let him fight, let him strike straight from the shoulder. Then only, when he has gained the power to resist, will non-resistance be a virtue."
Practice of non-violence should not prevent us from telling others when something they have done is not acceptable. Nor should we let others mistake our non-resistance for weakness. A devotee asked Sri Ramakrishna, "Sir, if a wicked man is about to do harm, or actually does so, should we keep quiet then?" The Master replied: "A man living in society should make a show of tamas to protect himself from evil-minded people. But he should not harm anybody in anticipation of harm likely to be done him....You must hiss at wicked people. You must frighten them lest they should do you harm. But never inject your venom into them. One must not injure others."
Spiritual aspirants cannot afford to nurture hatred or vengefulness toward anyone. Spiritual progress is not possible when these negative emotions hold sway over us. We can free ourselves from them by cultivating love for others and praying for their welfare.